In one prior art screwdriver, the screwdriver has a handle, a torsion unit, an actuation unit, a stopper, and a cover. The tension unit has a spring and a slide block. The spring is received in a receiving space of the handle. A periphery of the sliding block has a track corresponding to the guide groove of handle. The slide block is installed on the track and is stopped by the spring. The actuation unit has an actuate element and a driving rod which are integrally formed. One end of the driving rod is formed as a driving end which passes through the through hole of the slide block, and the holes of the tension unit and the handle. The actuate element is installed with push surface corresponding to a push surface of the slide block. The driving rod is formed with a positioning portion corresponding to a surface of the actuation unit. Thereby rotating the cover will set the twisting force applied.
However above mentioned structure cannot show the twisting force permissible. Thus the user can not know the set tension value easily. Furthermore, each time the screwdriver is used, the cover must be set again. Thus, the operation is inconvenient. Furthermore, only one end of the cover has the function of adjusting the twisting force. The effect is small.